Cmax (peak concentration)
Cmax is the highest drug concentration reached after a dose. At steady state it is written Cmax,ss. It matters for effects that depend on the peak, such as the first peak after a first dose.
Definition
Cmax is the highest concentration observed or predicted after a dose; the time it occurs is Tmax. At steady state it is Cmax,ss. For an infusion, the peak is at the end of the infusion, so a longer infusion lowers Cmax.
Dose, volume and peak
After an intravenous bolus the initial concentration is roughly the dose divided by the central volume of distribution, so the peak scales with dose and falls with a larger volume. Lengthening the dosing interval at a constant dose rate raises Cmax and lowers Ctrough.
On this site
The anti-PD-1 analysis used the median steady-state peak at the highest tested dose (10 mg/kg every 2 weeks) as an upper limit; the peak was rarely a problem except for nivolumab 480 mg every 4 weeks in the lightest patients. The tarlatamab analysis shows the peak rising when the interval is lengthened at constant dose intensity.
Frequently asked questions
Why does a longer infusion lower Cmax?
The drug enters more slowly while elimination continues, so the highest concentration reached is lower.
Is a high Cmax always a safety problem?
No. Whether a peak matters depends on the toxicity concerned and the drug; it has to be established separately from the exposure comparison.